The field of this invention is input circuits for digital controllers such as a programmable controller and more particularly input circuits for a digital controller for converting an a.c. input voltage to a logic level voltage.
In industrial environments, digital programmable controllers are commonly employed to control various devices, such as motors, relays, solenoids, or the like in a particular sequence in accordance with certain sensed conditions. The sensed condition may be the contact state of a switch, the resistance of a photoelectric cell or the level of an a.c. voltage. When the programmable controller is to control a motor for example in accordance with a sensed a.c. voltage, it is usually desirable to isolate the a.c. voltage from the programmable controller to avoid damaging of programmable controller components. To achieve isolation between the programmable controller and the sensed input voltage, an input module or circuit is employed to convert the sensed a.c. voltage into a logic level signal, which is then supplied to the input of the programmable controller which controls the motor or other device in accordance with the level of the logical voltage.
A.C. input circuits for converting a sensed a.c. voltage into a logic level voltage are known in the art. Examples of such circuits may be found in U.S. Pat. Nos. 3,691,403, 3,992,636 and 4,079,272. A typical a.c. input circuit such as described in the above identified patents includes a rectifier circuit for rectifying the sensed a.c. input voltage to provide a d.c. voltage whose magnitude varies directly with the magnitude of the sensed a.c. voltage. An optical isolator has its light emitter coupled, sometimes in series with a resistance to limit current to a predetermined value, across the rectifier output terminals so as to be energized therefrom. The photoconductive semiconductor of the optical isolator is rendered conductive by the light emitting diode when the d.c. output voltage of the rectifier becomes of sufficient magnitude to excite the light emitter. One or more Zener diodes may be coupled in series with the light emitter of the optical isolator across the rectifier output to set the voltage level at which the logic state of the opto-isolator changes.
Prior art input circuits for programmable controllers are subject to a form of "contact bounce", that is to say, the logical level of the output signal of the optical isolator tends to vacillate when the sensed a.c. input voltage across the rectifier input terminals varies slightly about the threshold voltage necessary to excite the light emitter of the optical isolator. As may be expected, this vacillation is very undesirable as it usually leads to a faulty logic level input to the programmable controller which then results in error in the control of the various output devices controlled by the programmable controller. Phrased in a different way, prior art input circuits for programmable controllers lack significant hysteresis characteristics which would prevent contact bounce due to small variations of the input voltage.